Organometallics 1994,13, 1178-1189
1178
Mechanistic Study of the Carbonylation of Nitrobenzene Catalyzed by the [Rh(CO)J/Nitrogen Base System. X-ray 1
Structure of [PPN][Rh(C0)20N( C6H&12)C(O)O] Fabio Ragaini and Sergio Cenini' Dipartimento di Chimica Inorganica, Metallorganica e Analitica and CNR Center, via G . Venezian 21, 20133 Milano, Italy
Francesco Demartin Istituto di Chimica Strutturistica Inorganica and CNR Center, via G. Venezian 21, 20133 Milano, Italy Received August 23, 199P
[PPN] [Rh(C0)41 (1) reacts with ArNOz or ArNO to yield the same product (Ar = Ph, 2; Ar = 3,4-C1&6H3, 3). The reaction of 1 with nitrosoarenes also produces the corresponding azoxyarenes. The product was structurally characterized in the case of Ar = 3,4-dichlorophenyl and was shown to be [ P P N I [ R ~ ( C ~ ) Z ~ N ( C ~ H ~ C ~(3). Z ) CCrystal ( O ) ~ ]data for 3: C46H33ClzN205PzRh.C4Hs0,Mr = 989.64, triclinic, space group Pi (No. 2), a = 10.798(4) A, b = 9.889(6) A, c = 21.648(6) A, a! = 89.77(1)', 0 = 96.00(1)', y = 92.71(1)', V = 2296(2) A2,2 = 2, D, = 1.431 g ~ m - Mo ~ , K a radiation (A = 0.710 73 A), ~ ( M Ka) o = 6.00 cm-l, R = 0.037 (R, = 0.047) for 5299 independent reflections having I > 3 4 ) . Both of the reactions proceed through electron transfer to the nitroso or nitro derivative. Reaction of 2 and 3 with MeOH and CO produced the corresponding amine or the methyl carbamate, depending on the experimental conditions. The reaction of 3 with MeOH/CO was faster in the presence of nitrogen bases, and aniline was shown to be an intermediate in the synthesis of the carbamate. When the same reaction was performed in the presence of a large excess of nitrosobenzene, the two isomeric mixed azoxybenzenes 3,4-C1&6H3N(O)=NPh and 3,4-Cl&H3N=N(O)Ph were the largely dominant products, which strongly suggests the intermediate formation of a nitrene complex. Introduction Carbonylation of organic nitro compounds is a reaction of high potential synthetic and industrial interest, since many products can be obtained from nitro compounds and CO in a single step, including amides, amines, oximes, ureas, carbamates, isocyanates, and indo1es.l Ureas and carbamates are important final products and intermediates in the synthesis of pesticides and fertilizers, and monoand diisocyanates are important intermediates in the manufacturing of pesticides, polyurethane foam plastics, synthetic leather, adhesives, and coatings. The classical method for producing isocyanatesrequires the intermediate reduction of a nitro compound to the correspondingamine, followed by reaction with phosgene. However, phosgene is a very toxic and corrosive material, and an enormous effort has been applied to the development of phosgene-free routes to isocyanates. Among these, the carbonylation of nitro compounds, particularly aromatic ones, represents one of the more interesting alternatives, but the direct carbonylation of nitro compounds to the corresponding isocyanates has proved to be a difficult reaction. However, in the presence of an alcohol, carbamates can be obtained more easily with high selectivity (eq 1).If the isocyanate is the desired product, this can be obtained (at least for R = aryl) by thermal cracking Abstract published in Aduance ACS Abstracts, February 1, 1994.
(1) Cenini, S.; Pizzotti, M.; Crotti, C. Metal Catalyzed Deoxygenation
Reactions by Carbon Monoxide of Nitroso and Nitro Compounds. In Aspects of Homogeneous Catalysis; Ugo, R., Ed.; D. Reidel: Dordrecht, The Netherlands, 1988; Vol. 6, pp 97-198.
Q276-7333/94/2313-1178$04.50/0
RNO,
-
+ R'OH + 3CO
RNHCOOR'
+ 2C0,
(1)
of the carbamate (eq 2). Recently, Liu and Cheng2 and A
RNHCOOR'
RNCO
+ R'OH
(2)
some of us3 independently reported the use of [PPNI [Rh(CO)rl (1; PPN+ = (PPh&N+) as a catalyst for the carbonylationof nitrobenzene to alkyl phenylcarbamates. We also found that bpy (2,2'-bipyridyl) is a good cocatalyst for this reaction. The main byproducts were aniline and azoxybenzene,although trace amounts of azobenzene were also present. The amount of azoxybenzene produced was highly dependent on the concentration of nitrobenzene in the catalytic solution, and none was formed at low concentrations.3 In this paper we report the results of a mechanistic study on this catalytic system. A preliminary account of part of this work has been p ~ b l i s h e d . ~ Results and Discussion Reactions of [PPN][Rh(CO)4] ( 1 ) with bpy, ArN02, a n d ArNO. We first checked the possible reaction of 1 with bpy to afford a complex such as [PPNl[Rh(C0)2(bpy)]. No reaction was observed in refluxing THF or (2) Liu, C.; Cheng, C. J. Organomet. Chem. 1991,420,119.
(3) Ragaini,F.;Cenini,S.;FumageJli,A.;Crotti,C.J.Orga"et.Chem. 1991,428, 401. (4) Ftagaini,F.;Cenini,S.;Demartin,F.J. Chem. SOC.,Chem. Commun. 1992, 1467.
0 1994 American Chemical Society
Carbonylation of Nitrobenzene
Organometallics, Vol. 13, No. 4, 1994 1179
c1
Figure 1. Ortep view of the [Rh(CO)~0N(C~H~C1~)C(0)01anion. Thermal ellipsoids are drawn at 30% probability.
acetone, whereas rhodium metal and [RhClz(CO)zI-were respectively formed in refluxing toluene and dichloroben~ e n e .Under ~ the same conditions used in catalytic reactions (200 "C, Pco = 60 atm, THF), some as yet unidentified compounds (probably cluster complexes)were formed, but they reacted with nitrobenzene much more slowly than did 1 and cannot be intermediates in the catalytic cycle. As it seems possible to exclude the direct interaction of 1 by bpy, we examined the reaction between 1 and nitrobenzene. A smooth reaction was observed at 40 OC, affording a complex (2) showing three bands in the IR spectrum a t 2045 (s), 1967 (vs), and 1635 (m) cm-1 (THF). The same product was obtained by the use of nitrosobenzene in place of nitrobenzene, but 3 equiv of nitrosobenzene was necessary and azoxybenzene was also formed.6 The reaction with nitrosobenzene was much faster and was complete in less than 3 min at room temperature. The use of smaller amounts of nitrosobenzene gave the same product together with unreacted 1. No intermediate was observable by IR spectroscopy during either of these two reactions. ( 5 ) In our first paper on this topic: we tentatively formulated the product of the reaction of 1 with bpy in dichlorobenzene as [Rh(CO)t-
(bpy)]. However we later discovered, as reported here, that this complex was indeed [RhCl&O)zl-. Therefore, the original proposal must be considered erroneous. Also, the previously reported reaction of this complex with nitrobenzene, to yield a still uncharacterized compound, is a reaction of [RhC12(C0)21-. (6) An additional small absorption of varying intensity at 1672 cm-1 in the IR spectrum of the reaction mixture was observed when nitroeobenzeneor other nitrosoareneawere used as starting materials. When excess ArNO was added to our reaction mixture, the absorption did not disappear. Therefore, it cannot be attributed to any intermediate in the formation of 2. In our f i t paper on this topic: we reported that two different products (at that time both uncharacterized) were formed by reaction of 1withnitroso- and nitrobenzene. The products were considered to be different on the basis of slightly different IR spectra recorded on an old IR spectrophotometer. However, when the same reactions were examined using the more precise and reproducible FT-IR spectrophotometer used in the course of this study, the two product spectra were identical. Additional evidence for the identity of the products of the two reactions comes from their mass spectra (vide infra).
Unfortunately, 2 was not stable enough to allow purification, but an analogous, more stable complex (3) was obtained by use of 3,4-dichloro-l-nitrobenzene, and its structure was solved by X-ray diffraction (Figure 1). The nitro group of the starting organic compound has been reduced to a nitroso moiety by a coordinated CO, but the COZmolecule thus resulting has not left the coordination sphere of the metal. It should be noted that COZhas surely rearranged from its original position, in which the carbon atom was necessarily bound to rhodium. This is the first time that a metallacycle of this kind has been observed during a deoxygenation reaction of nitrobenzene, although two similar metallacycles have been previously obtained by reaction of COz with nitroso complexes.798 In the present case, however, the possibility that COZis delivered into the solution and then taken up again can be excluded, since no appreciable variation in the outcome of the reaction was observed when nitrogen was vigorously bubbled through the solution during the reaction. Crystallographic details and selected interatomic disI
tances and angles within the [ R ~ ( C O ) Z O N ( C ~ H ~ C ~ ~ ) 1
C(0)Ol- anion of 3 are reported in Tables 1-3. A perspective view of the anion is shown in Figure 1. Crystals
I
of compound 3 contain P P N + cations, [Rh(CO)Z1
ON(C~H~C~Z)C(O)O]anions, and THF molecules packed in the crystal lattice in the molar ratio 1:l:l; interionic and intermolecular contacts are not shorter than the normal van der Waals interactions. The rhodium atom displays the expected square-planar coordination. Displacements (A) from the average coordination plane are as follows: C(1), -0.044(5); C(2), 0.044(5); 0(11), -0.043(3);0(12),0.044(3). Also, the metallacycle is slightly folded with the following deviations (A) from the average (7) Cenini, S.; Porta, F.; Pizzotti, M.; Crotti, C. J. Chem. SOC.,Dalton Trans. 1985,163. (8) Dahlenburg, L.; Prengel, C. Inorg. Chim. Acta 1986, 122, 55.
1180 Organometallics, Vol. 13, No. 4, 1994
Ragaini et al.
Table 1. Crystal Data for
Table 2. Fractional Atomic Coordinates for
[PPNIdh(CO)2ON(C&ClCIs)C(O)OfCfiO formula fw cryst syst
[ P P N I d h ( C O ) ~ O N ( C ~ ~ C l ~ ) C ( O ) O (3) ]~C~O
(3)
C4~H~~C12N20sP2Rh.C.HsO 989.64 triclinic
atom
X
Y
z
c(222j C(223) C(2241 ~ ( 2 2j5 C(226) C(231) C(232) C(233) C(234) C(235) C(236) C(900) O(901) C(902) C(903) C(904)
0.382 14(3) 0.3830(2) 0.0901( 1) -0.13366(8) 0.06701(9) 0.2245(4) 0.5307(3) 0.4895(2) 0.2848(3) 0.5 141(3) -0.001 2(3) 0.3527(3) 0.2827(5) 0.4741(4) 0.4585(4) 0.2925(4) 0.3579(4) 0.2965(4) 0.1694(4) 0.1035(4) 0.1644(4) -0.1 280(3) -0.0157(4) -0.0114(4) -0.1 170(5) -0.2285(4) -0.2348(4) -0.173 l(3) -0.0836(4) -0.1 132(5) -0).2312(5) -0.3201(4) -0.2924(4) -0.2608( 3) -0).2877(4) -0).3758(5) -0.4395(5) -0).4138(5) -0.3248(4) 0.2104(3) 0.2725(4) 0.3884(4) 0.4438(4) 0.3829(4) 0.2657(4) 0.1086(3) 0.2116(4) 0.2431(4) 0.1727(4) 0.0701(4) 0.0386(4) -0.0255(4) -0.0408( 4) -0.128 l(5) -0.1993(5) -0.1827(5) -0.0960(4) 0.234( 1) 0.1463(8) 0.1845(9) 0.3027(8) 0.3430(8)
0.147 15(4) -0.1 109(2) -0.0824(2) 0.32860(9) 0.5453( 1) 0.359 l(5) 0.1753(4) 0.01 1l(3) 0.1 11l(3) -0.1062(4) 0.4041(3) 0.0323(3) 0.2759(6) 0.1664( 5 ) -0.0266(4) 0.0021(4) 4.0376(4) -0.0631(4) -0.0496(5) -0.0123(5) 0.0157(4) 0.1836(4) 0.1498(4) 0.0357(5) -0.0446(4) -0.0126(4) 0.101 5(4) 0.2689(4) 0.2742(5) 0.2268(6) 0.1764(5) 0.1703(5 ) 0.21 70(5) 0.4276(4) 0.4371(4) 0.5239(6) 0.5984(6) 0.591 l(5) 0.5058(5) 0.5179(4) 0.6189(4) 0.5975(5) 0.4760(5) 0.3753(5) 0.3954(4) 0.6289(4) 0.7201(4) 0.7804(4) 0.7543(4) 0.6657(5) 0.6022(4) 0.6563(4) 0.6329(5) 0.7040(6) 0.7956(5) 0.8210(5) 0.7520(4) 0.299( 1) 0.387( 1) 0.501( 1) 0.493( 1) 0.358( 1)
0.1 1264(2) 0.45657(6) 0.45082(6) 0.19726(4) 0.21839(4) O.OSSO(2) 0.0052(2) 0.1583(1) 0.1845(1) 0.2468(2) 0.2012(2) 0.2285(2) 0.0769(2) 0.0465(2) 0.2124(2) 0.2815(2) 0.3368(2) 0.3883(2) 0.3864(2) 0.3312(2) 0.2796(2) 0.1482(2) 0.1278(2) 0.091O(2) 0.0755(2) 0.0952(2) 0.1320(2) 0.27 19(2) 0.3224(2) 0.3793(2) 0.3864(2) 0.3367(2) 0.2795(2) 0.1652(2) 0.1013(2) 0.0766(2) 0.1 144(3) 0.1779(3) 0.2032(2) 0.2655(2) 0.3024(2) 0.3329(2) 0.3270(2) 0.2905(2) 0.2597(2) 0.1490(2) 0.1503(2) 0.0964(2) 0.0408(2) 0.0389(2) 0.0928(2) 0.2583(2) 0.3203(2) 0.3480(2) 0.3 145(3) 0.2543(3) 0.2255(2) 0.461 2(6) 0.4625(5) 0.4804(5) 0.5152(5) 0.5068 (6)
Pi 10.798(4) 9.889(6) 21.648(6) 89.77( 1) 96.00( 1) 92.7 1(1) 2296(2) 2 1.431 6.00 0.93
p?;: group b (A) c (A)
a (deg)
B
Y (deg)
v (A3)
Z
&I, (g ~ m - ~ ) ~r (cm-I)
min transmissn factor scan mode w scan width (deg) e range (deg) reciprocal space explored no. of measd rflns no. of unique obsd rflns with I final R and Rw indices’ no. of variablcs GOFb
w
+
1.3 0.35 tan 6 1-25
+h,*:k,*l
> 341)
8055 5299 0.037,0.047 559 1.495
[ZW(FO- k W ) z / C ~ F o 2 ] 1*/ 2GOF . 0 R = [E(Fo- WJ)/EF,];Rw = [ Z N F o - YFJ)2/(Nobwmtiol. - NwiatdI1/’: w 1/(dFo))’, 4 F o ) = [a2(f) + (0.041)2]1/2/2F&p.
plane: Rh,-O.M8(1);0(11),0.040(3);0(12),0.069(3);N(1), -0.061(3); C(3), O.OOO(4). The C(l1-16) phenyl ring is twisted about the N(l)-C(ll) bond with respect to the metallacycleplane, with a C(3)-N(l)-C(ll)-C(12) torsion angle of 32O. The pattern of bond distances of the metallacycle and neighbor atoms is indicative of a predominant double-bond character for the C(3)-0(13) interaction and of delocalized r electron density through the remaining atoms. The reactions of nitro- and nitrosoarenes with 1 can be now written as shown in eqs 3 and 4.
L
J
anion of 2, Ar = Ph anion d 3, Ar = 3,4-Ci&g13
[PpNI[Rh(CO)4] 1
+ 3PhNO
THF
0 2
t
+ PhN=NPh
(4)
Reactions 3 and 4 can be performed under either a dinitrogen or CO atmosphere, but yields were consistently higher in the latter case. Despite the fact that 2 and 3 are the only carbonyl-containing products observed (see, however, ref 6),some dark material was also formed during the reactions. Reactions 3 and4 both proceed through an intermediate electron transfer from 1to the nitro or nitroso compound. This was evidenced by performing reaction 4 (employing a large excess of nitrosobenzene) inside an EPR tube, equipped with a side reservoir which allowed mixing of the reagents just before recording the spectra. Such a reaction gave a strong EPR signal at room temperature that is attributed to the radical anion of nitrosobenzene dimer on the basis of the typical 45-line splitting (a triplet, Jt = 14.4 G, split into quintuplets, Jp= 3.0 G, split into
triplets, Jt= 1.0 G)previously reported in the literature.9 No ESR signal could be observed when reaction 3 was performed with nitrobenzene, probably because the reaction is too slow at room temperature. However, astrong signal was immediately apparent when 3,4-dichloronitrobenzene was employed. The splitting could be best resolved by cooling the solution to 0 “C 2 min after mixing of the reagents, and the signal consists of a 1:l:l triplet (Jt= 8.8 G)split into doublets ( J d = 3.9 G).This splitting is clearly indicative of a nitrogen-centered radical. How-
Carbonylation of Nitrobenzene
Organometallics, Vol. 13, No. 4, 1994 1181
Table 3. Selected Interatomic Distances (A)and Angles I
I
(deg) for [PPNIR~(CO)~ON(C~~C~~)C(O)O~~C~H~O (3) [Rh(CO)dRh-O( 11) Rh-O( 12) Rh-C(l) Rh-C(2) Cl(l)-C( 13) CI(2)-C( 14) O(1)-C( 1) 0(2)-C(2) 0(11)-C(3) O(12)-N( 1) 0 ( 1 l)-Rh-0(12) O(11)-Rh-C( 1) O(1l)-Rh-C(2) O(12)-Rh-C( 1) 0(12)-Rh-C(2) C( l)-Rh-C(2) Rh-O(11)-C(3) Rh-O( 12)-N(1) O(12)-N( 1)-C(3) O(12)-N( I)
